The invention relates to a method for producing a hot strip from a transformation-free ferritic steel, wherein a melt is cast into a roughed product and the latter is then rolled into a hot strip.
Transformation-free ferritic steels cannot be produced by using the common continuous casting route, i.e. continuous casting of the melt into a slab or thin slab which is rolled either in-line or separately into a hot strip, with the required properties.
The reasons for that reside in the fact that the slab or thin slab, produced by continuous casting, has macro segregations and forms shrink marks. Moreover, the roughed product has a very coarse grain and casting with casting powder poses problems because of the high aluminum content of the ferritic steel.
DE 100 60 948 C2 discloses a production of hot strips from steel having a high manganese content with 12 to 30 weight-% of manganese and up to 3.5 weight-% of each of aluminum and silicon in such a way that the steel melt is cast in a double-roller casting machine to form a roughed strip close to the final dimensions with a thickness of up to 6 mm, and subsequently the roughed strip is hot rolled continuously preferably in a single pass.
The stated upper limit for the thickness with 6 mm cannot be achieved with existing facilities. The maximum thickness that can actually be adjusted is typically 4 mm, in exceptional cases maximal 5 mm.
This known method has the advantage that macro segregations are reduced, shrink marks are suppressed, and the problem associated with casting powder is not relevant.
It is, however, disadvantageous that the small starting thickness of the hot strip permits only a small hot deformation degree during rolling, when a thickness of 2-3 mm of the hot strip is desired.
This thickness range, for example, is however of interest for the use of the hot strip as lightweight component in the exhaust tract of motor vehicles on the one hand. On the other hand, a cold strip with a thickness of, for example, 1.0-1.8 mm can be produced from a hot strip of a thickness of 2-3 mm at a degree of deformation of 40-50% and can again be used, for example, in the exhaust tract of motor vehicles. A small hot deformation degree means, however, coarse grain which adversely affects ductility and thus the formability of the hot strip.